AbstractBasic helix–loop–helix (bHLH) transcription factors play important roles in cell type specification and differentiation during the development of the nervous system. In this study, we identified a chicken homolog of Atonal 8/ath6 (Cath6) and examined its role in the developing retina. Unlike other Atonal‐family proneural genes that induce neuronal differentiation, Cath6 was expressed in stem cell‐like progenitor cells in the marginal region of the retina, and its overexpression inhibited neuronal differentiation. A Cath6 fused with a VP16 transactivation domain recapitulated the inhibitory effect of Cath6 on neuronal differentiation, indicating that Cath6 functions as a transcription activator. These results demonstrate that Cath6 constitutes a unique member of the Atonal‐family of genes in that it acts as a negative regulator of neuronal differentiation. Developmental Dynamics 239:2492–2500, 2010. © 2010 Wiley‐Liss, Inc.